The first step in assessing the cause of canopy chlorosis and decline in an orchard is mapping the distribution of the symptoms. If a pattern of chlorosis is similar across irrigation lines, then the cause of the problem may be related to over- or under-watering. Two scenarios present themselves regularly during summer farm calls: a) terminal tree chlorosis, and b) within row tree chlorosis (Figures 1 and2). 

Terminal Tree Chlorosis.  In some orchards, the terminal tree along the irrigation line may become chlorotic and decline in advance of mortality. If terminal tree chlorosis is a trend throughout the orchard, it is worth assessing the sprinkler distribution at the end of the irrigation lines. In some orchards, the terminal tree is outfitted with a sprinkler that is not shared with a neighboring tree (Figure 2A). This terminal tree receives 1.5 x the amount of water as the other ‘healthy' trees down the irrigation line. In an otherwise adequately-irrigated orchard, these terminal trees are over-irrigated and develop chlorosis and decline. Sometimes the terminal sprinkler is positioned adjacent to the trunk (Figure 3), resulting in direct wetting of the trunk, a condition that predisposes the tree to Phytophthora infection, particularly when surface water is utilized.

Correcting terminal tree chlorosis:  To correct the over-irrigation of the terminal tree, the microsprinkler head can be changed to a lower flow rate.  Sprinklers should be placed away from the base of trees to prevent direct contact of the trunk with the stream of water. Additionally, when replanting dead or declining trees at the end of rows, consider that the irrigation needs of the replant are considerably lower than that of the neighboring older tree in the row.

Within-row chlorosis. If canopy chlorosis is consistent throughout the orchard, but terminal trees appear healthy, assess the distribution of sprinklers around the terminal tree in comparison to the trees along the irrigation line. If the terminal tree receives less water (Figures 1B and 2B) than adjacent chlorotic trees, consider the potential that the orchard, as a whole, is over-irrigated. To test this hypothesis, growers and orchard managers can use a pressure chamber to assess the midday stem water potential of the trees. Almond trees maintained from ^-6-^-10 bar are under low water stress, but may be more susceptible to disease. Maintenance of almonds at ^-10-^-14 bar (mild stress) from mid-June through hull split, minimizes risk of disease (ie. hull rot) and supports shoot growth. For information on use of a pressure chamber for enhanced irrigation management of almond, walnut and prune, download UC ANR Publication #8503 (http://ucanr.edu/datastoreFiles/391-761.pdf).

Correcting within-row chlorosis:  If the orchard at large is over-irrigated, a change in the overall irrigation strategy is warranted. A combination of pressure chamber use to measure tree water stress, and consideration of weekly crop transpiration may enhance irrigation scheduling. The California Department of Water Resources and UCCE have teamed up to provide Weekly ET Reports to agricultural water users to assist with irrigation scheduling. The reports include water use information for a variety of crops including almonds, pistachios, walnuts, grapevines, citrus, and stone-fruit of mature bearing age. Adjusted on a weekly basis, water use estimates account for the changing growth stage and weather conditions at the Madera, Parlier, Lindcove, Stratford, Panoche, and Five-Points CIMIS weather stations. Each report gives crop-specific evapotranspiration (ETc, total crop water use including soil evaporation) estimates for the previous and coming week. To learn how to use these reports, please refer to the following article:  http://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=26858.  Crop ET reports can be found online (ie:  http://cetulare.ucanr.edu/Agriculture782/Custom_Program911/). 

Over the past decade, combined nut acreage in Tulare and Kings Counties has more than doubled, resulting in an expansion of the labor force needed to harvest the >250,000 acres of nuts locally. The annual Nut Harvest Safety Training co-hosted by the Kings County Farm Bureau, the Tulare County Farm Bureau, and UCCE Tulare and Kings Counties attracted over 150 attendees to the Kings County Fairgrounds on July 11, 2018. The program was conducted in both English and Spanish, with Walter Martinez and Gabriel Torres, UCCE Tulare County, serving as translators. The program was additionally sponsored by the American Pistachio Growers.

Dr. Farzaneh Khorsandi Kouhanestani, a new CE Specialist at UC Davis in Biological and Agricultural Engineering joined the program with a talk on equipment safety. Martha Sanchez, Department of Pesticide Regulation presented on chemical safety. Officers Chris Tuttle and Gabriel Perez, California Highway Patrol, discussed road safety, and Gumaro Castellanos, Nationwide, presented on heat illness prevention. Francisca Camarena and Mireya Gutierrez of Family HealthCare Network were available during registration to measure attendees' blood glucose level and blood pressure. 

It is not often that we observe cankers caused by Phytophthora on pistachio in CA; however, the unique situation observed on a recent farm call is worth recounting. The decline and mortality (Figure 1A) of approximately 20 trees in a Tulare County pistachio block of ‘Golden Hills' on PG1 rootstock (a selection of Pistacia integerrima) was unusual simply because we generally find PG1 rootstock to be resistant to Phytophthora. In fact, the rootstock did not exhibit any symptoms of cankering and the cambium appeared healthy (Figure 1B).  Rootstock suckers were actively growing, suggesting that the pathogen was not infecting the roots or crown of the tree (Figure 1A). The scion, however, exhibited extensive bleeding (Figure 1C) and gummosis (Figure 1D), which is typical of Phytophthora infection on Pistacia vera.

The diagnosis of this problem was possible only after gathering information from the grower on orchard management history. Recently the grower had changed the microsprinkler nozzles to a higher flow rate nozzle (14 gallon/hr). The stream of water from the microsprinkers was wetting the trees above the graft union. Additionally, the grower had recently utilized surface water (not ground water) for irrigation. Surface water has a high probability of Phytophthora contamination. The combination of the change to higher flow rate sprinklers, wetting of the scion wood, and risk of introduction of Phytophthora to the orchard, all suggested that the cankering, canopy decline, and mortality is related to Phytophthora infection. After sampling the canker tissue and incubating it in selective medium, the presence of Phytophthora sp. was confirmed in the tissue.

To prevent further incidence of Phytophthora in the orchard, the irrigation system needs to be changed to prevent water from splashing on the tree trunks. This may include both changing the nozzles to a lower flow rate, and moving sprinklers further from the base of trees. As long as the water does not touch the susceptible P. vera scion, the introduction of Phytophthora to the orchard in surface water is of minimal risk to orchard health.  In order to preserve the rootstock and enhance regeneration of productive, economically viable trees, the grower may opt to cut the scion off to a point below the graft union and re-graft onto the mature rootstock.

The grower may also opt to run products through the irrigation system that may assist with disease management.  Two examples of products that have been found to prevent Oomycete infection on pistachio and other crops include phosphites and Ascophyllum nodosum extract. Replicated trials using K-PHITE 7LP (Plant Food Company, Inc) and Acadian (Acadian Seaplants) have demonstrated that both product types offer protection of pistachio from infection by Phytopythium helicoides (another Oomycete causing root rot on pistachio and almond), and therefore may have value in mitigating future infections within the orchard. K-PHITE 7LP is labeled for management of Phytophthora on pistachio. Acadian, an extract of A. nodosum, is registered as a fertilizer; however, the efficacy of A. nodosum extract in mitigating plant diseases, including those caused by Oomycetes, is well documented in the scientific literature.

For more information on management of Phytophthora, visit UC IPM Online (www.ipm.ucdavis.edu).  Always read the label of the product being used, and note that all registered pesticides are not necessarily listed on the UC IPM Online website (http://www.ipm.ucdavis.edu) or in this newsletter.  Always check with certifier to determine which products are organically acceptable.

Evaluation of flowering and fruit set on almond allows for within-season assessment of orchard productivity; however, understanding the vegetative growth dynamics of almond allows growers to consider parameters affecting productivity years into the future.  Vegetative growth of almond has two main components: vegetative shoot growth and spur production. Vegetative shoot growth provides the overall architecture of the canopy, and spur production generates the tissues that give rise to the majority of fruit in subsequent seasons.  Both vegetative shoot growth and spur production are key components to the development of an economically sustainable and productive orchard.

Timing of vegetative growth.  All buds (vegetative and flower) are formed during the prior season. Because almond has one of the lowest chill requirements of permanent crops grown in California, the chill requirement is generally fulfilled by January 1. As temperatures increase, growth initiation is induced, and bud break ensues, with flower buds breaking in advance of vegetative buds. Vegetative shoot growth proceeds at a somewhat uniform rate throughout the season on young trees, but the durationof spur elongation is short and generally complete by April or early May.

Vegetative buds. On almond, vegetative buds can be distinguished from flower buds by shape. Flower buds are thick and oval; vegetative buds are pointy and triangular. On shoots, flower buds are generally formed on either side of a vegetative bud. On spurs, the apical bud is always vegetative (Figure 1A), and this bud can give rise to either further spur growth (Figure 2) or a vegetative shoot (Figure 1B). Spurs in positions with high light interception are more likely to give rise to vegetative shoots than new spur growth. 

Vegetative shoot growth. Vegetative buds may give rise to long vegetative shoots that support future spur production. During the early years of orchard establishment, long shoot growth is the main component of vegetative development on almond (Figure 3A, B, C). On mature trees, vegetative shoot growth occurs under conditions of low crop, high vigor, and in regions of the canopy where there is excessive light interception. Canopy regions with excess light include external/exposed areas and empty spaces resulting from broken limbs.

Spurs. Spurs are short, compact vegetative shoots, approximately 0.5-2 inches long (Figure 2). Spurs arise on vegetative shoots or on spurs produced in the prior season (Figure 2). Within a season, the duration of spur growth is generally short, with spur extension completed by April or early May. Spurs are always formed on the prior year's wood, and remain vegetative for 1-2 years prior to flowering. As a consequence, the process from vegetative shoot growth to spur production and flowering may take 4 seasons.

Spurs support approximately 80% of the total almond yield in a given year, yet only about 20% of the total spur population on a tree supports nut production each year.  The fact that only 1 in 5 spurs bear fruit in a season is explained by the dynamic status of spurs between years. A portion of spurs remain only vegetative in a given year (Figure 1C), whereas others may support 1-5 flowers that may develop into single fruit-bearing spurs or multiple fruit-bearing spurs (Figure 1D). Due to the reliance on a localized carbon economy, individual spurs tend to alternate bear, meaning that spurs that bear fruit one year tend not to flower or bear fruit the following year.

Comprehensive view on vegetative growth. In new almond plantings (Figure 3A), growers should expect the mainstay of vegetative growth to be production of long vegetative shoots (Figure 3B and C). Although the majority of the future crop is produced on spurs, it will take time for bearing spurs to be represented in the canopy.  Consider that spurs are produced on the prior year's wood and will remain vegetative for 1-2 years before entering productivity. Patience is needed as these vegetative spurs store carbohydrates to support future nut development.

Select References

Tombesi, S., Lampinen, B.D., Metcalf, S., DeJong, T.M. 2016. Yield in almond related more to the abundance of flowers than the relative number of flowers that set fruit. California Agriculture 71: 68-74. Online: https://doi.org/10.3733/ca.2016a0024

Lampinen, B.D., Tombesi, S., Metcalf, S.G., DeJong, T.M. 2011. Spur behavior in almond trees:  relationships between previous year spur leaf area, fruit bearing and mortality. Tree Physiology 31:  700-706. Online:  https://doi.org/10.1093/treephys/tpr069

Kester, D.E., Martin, G.C., and Labavitch, J.M. 1996. Growth and Development. In: W.C. Micke, Editor, Almond Production Manual. Oakland, California, University of California, Division of Agriculture and Natural Resources (pp 90-97).